Synaptic plasticity in the cerebellum is an important substrate of motor learning in supraspinal regions of the brain. Although a great deal of information is available concerning he cellular mechanisms which mediate long-lasting forms of synaptic plasticity in other regions of the nervous system, little is known about the cellular mechanisms which mediate synaptic plasticity in the cerebellum. Two forms of long-lasting change in the efficacy of the parallel fiber response have been identified: long- term potentiation and depression, which result from the conjunctive activation of climbing and parallel fiber inputs to cerebellar Purkinje cells. We propose to examine the cellular mechanisms which underlie these forms of synaptic plasticity using an in vitro preparation of the turtle brainstem-cerebellum. These preparation has been chosen because the unique anoxia resistance of turtle brain allows stable intracellular recording from Purkinje cells to be combined with selective stimulation of climbing and parallel fibers afferents. The goal of this proposal is to (i) characterize the physiological and pharmacological properties of climbing and parallel fiber inputs to Purkinje cells, (ii) to analyse the time course and cellular substrate of long-lasting changes in parallel fiber efficacy, and (iii) to develop a model of the cellular mechanisms of synaptic plasticity using both the intact brainstem-cerebellum and acutely dissociated Purkinje cells. To characterize the properties of climbing and parallel fiber inputs to Purkinje cells, the biophysical properties of synaptic responses will be compared with responses to pressure-ejected amino acid neurotransmitter candidates. Bath-applied pharmacologic antagonists will be employe to further determine the receptor subtypes mediating excitatory transmission to Purkinje cells. Synaptic plasticity in the cerebellar cortex will be examined by conjunctive stimulation of "on-beam" parallel fibers and the inferior olive. The time course of changes in the parallel fiber potential will be compared with changes in agonist sensitivity. The modulation of long- lasting changes by norepinephrine will also be examined. Receptor antagonists will be applied to elucidate which receptor subtypes are involved in the initiation and maintenance of these long-lasting changes. The conjunctive application of appropriate agonists to acutely isolated and intact. Purkinje cells will be employed to further define a model of the cellular mechanisms of synaptic plasticity. Using acutely dissociated, voltage-clamped Purkinje cells and "concentration jump" agonist application techniques, the long-lasting changes in receptor sensitivity hypothesized to underlie synaptic plasticity will be mimicked. The application of agents in the extra- and intracellular solutions which modify second messenger systems will be employed to determine the intracellular events which mediate conjunctive stimulation. The results of this study should provide important new insights into the cellular mechanisms which underlie long-lasting changes in synaptic transmission in the cerebellar cortex. These mechanisms are likely of fundamental importance in understanding the cellular substrates of motor learning.